Skin is made up of several layers of cells which coat and protect the keratin and collagen fibrous proteins that form the skeleton of its structure. The outermost of these layers, referred to as the stratum corneum, is known to be composed of 25 nm protein bundles surrounded by 8 nm thick layers. Anionic surfactants and organic solvents typically penetrate the stratum corneum membrane and, by delipidization (i.e. removal of the lipids from the stratum corneum), destroy its integrity. This destruction of the skin surface topography leads to a rough feel and may eventually permit the surfactant or solvent to interact with the keratin, creating irritation.
It is now recognised that maintaining the proper water gradient across the stratum corneum is important to its functionality. Most of this water, which is sometimes considered to be the stratum corneum's plasticizer, comes from inside the body. If the humidity is too low, such as in a cold climate, insufficient water remains in the outer layers of the stratum corneum to properly plasticize the tissue, and the skin begins to scale and becomes itchy. Skin permeability is also decreased somewhat when there is inadequate water across the stratum corneum. On the other hand, too much water on the outside of the skin causes the stratum corneum to ultimately sorb three to five times its own weight of bound water. This swells and puckers the skin and results in approximately a two to three fold increase in the permeability of the skin to water and other polar molecules.
Hair consists of many of the same constituents as the stratum corneum. The outermost region of cells forms a rather thick chemically resistant protective coating enclosing the hair fibre which is called the cuticle. The surface of the cuticle is covered with a thin layer called the epicuticle which is thought to contain lipids and protein. The cuticle envelopes the cortex cells which comprise the major part of the fibre mass. Keratinization takes place in the cortex to build stability into the hair structure.
Thus, a need exists for compositions which will assist the stratum corneum and hair cuticle in maintaining their barrier and water-retention functions at optimum performance in spite of deleterious interactions which the skin and hair may encounter in washing, work, and recreation.
Conventional cosmetic cream and lotion compositions as described, for example, in Sagarin, Cosmetics Science and Technology, 2nd Edition, Vol.I, Wiley Interscience (1972) and Encyclopaedia of Chemical Technology, Third Edition, Volume 7 are known to provide varying degrees of emolliency, barrier and water-retention (moisturizing) benefits. However, they can also suffer serious negatives in terms of skin feel (i.e. they often feel very greasy on the skin) as well as having poor rub-in, absorption and residue characteristics. In the case of hair-care compositions they can also suffer from resoiling negatives.
The present invention therefore provides skin- and hair-care cosmetic compositions which provide improvements in moisturization, absorption, residue, tackiness, skin feel and skin care characteristics and which in particular provide improved short and longer term moisturizing effectiveness, while at the same time avoiding depositing oily residues on the skin.